In recent years, a multicarrier system, more specifically, an orthogonal frequency division multiple access (Orthogonal Frequency Division Multiple Access, OFDMA) system is drawing attention, as a wireless communications system.
When the OFDMA is applied in a downlink in a mobile communications system, throughput of the system can be improved by allocating a mobile station with a good propagation characteristic to each subcarrier.
FIG. 3 is a diagram showing a configuration of a wireless communications system formed of a base station 10 and mobile stations 11 and 12.
The base station 10 performs mobile station allocation for each frequency block formed of one or a plurality of subcarriers and according to the propagation characteristic of a downlink.
The base station 10 transmits a pilot signal to the mobile stations 11 and 12 through the downlink in order to obtain information on the propagation characteristic for each subcarrier in the downlink, for example. Each of the mobile stations 11 and 12 measures reception quality of the pilot signal, and transmits to the base station through an uplink the measured reception quality as channel quality information.
Normally, the channel quality information is transmitted for each frequency block rather than an individual subcarrier, in order to reduce transmission amount of the channel quality information. Allocation of the mobile stations 11 and 12 is also performed for each frequency block.
FIG. 4 is a diagram showing a configuration example of the base station 10.
A radio resource allocation control unit 23 determines allocation of the mobile stations 11 and 12 based on the channel quality information on each of the mobile stations 11 and 12 for each subcarrier and instructs the allocation to a transmission unit 22.
The transmission unit 22 allocates downlink data for the mobile stations 11 and 12 to subcarriers according to the instruction of the radio resource allocation control unit 23 and transmits the downlink data through a transmission antenna 21.
Such a radio resource allocation technique is disclosed in “Wireless communications System”, IEICE Technical Report, Vol. 104, No. 440, pp. 31-36 (Non-Patent Document 1), for example. Non-Patent Document 1 discloses that, using either an instantaneous value (an “instantaneous received SINR”) of a received signal to interference plus background noise power ratio (Signal-to-Interference plus background noise power ratio, SINR) in each frequency block, or a value obtained by normalizing the instantaneous received SINR by an average received SINR as an evaluation criterion for allocation, a mobile station with the largest evaluation value is allocated to each frequency block, thereby allowing improvement in the throughput. The average received SINR is obtained by temporally averaging the instantaneous SINRs.                [Patent Document 1] JP Patent Kokai Publication No. JP-P-2006-094005A        [Patent Document 2] JP Patent Kokai Publication No. JP-P-2006-191533A        [Non-Patent Document 1] Nagata et al (other four), “Wireless communications System”, IEICE Technical Report, Vol. 104, No. 440, pp. 31-36, 2004        [Non-Patent Document 2] Teng et al (other three), “Wireless communications System”, IEICE Technical Report, Vol. 102, No. 550, pp. 83-88, 2003        [Non-Patent Document 3] M. Morretti et al (other one), Proc. 2006 IEEE 63rd Vehicular Technology Conference (VTC2006-Spring), Vol. 5, pp. 2109-2113, 2006        [Non-Patent Document 4] Y. J. Zhang et al (other one), IEEE Trans. Wireless Communications, Vol. 3, No. 5, pp. 1566-1575, 2004        [Non-Patent Document 5] W. Rhee et al (other one), Proc. 2000 IEEE 51st Vehicular Technology Conference (VTC2000-Spring), Vol. 2, pp. 1085-1089, 2000        
The following analyses are given by the present invention. The entire disclosures of the above mentioned Patent and Non-Patent Documents are herein incorporated by reference thereto.
FIG. 5 shows an example of channel quality information on a plurality of mobile stations in each of frequency blocks.
Generally, the channel quality information corresponds to a modulation and coding scheme (Modulation and Coding Scheme, MCS) which satisfies a reception error rate of a certain reference value or less. The channel quality information in FIG. 5 is indicated by MCS1 to MCS6, respectively.
FIG. 6 shows an example of allocation indices for the mobile stations in each of the frequency blocks.
In the case of the characteristic in FIG. 6, frequency blocks #1 to #5 and frequency blocks #10 to #14 are allocated to a mobile station 1, while frequency blocks #6 to #9 are allocated to a mobile station 3.
The MCSs shown in FIG. 5 are applied to the mobile stations for which allocation has been performed, for each frequency block.
The followings analyses are given by the present invention. The above mentioned patent documents and non-patent documents are incorporated herein by reference thereto.
In Non-Patent Document 1, a mobile station is allocated independently, for each frequency block as a unit. In each frequency block, an MCS (shown in FIG. 5) in accordance with the channel quality of the allocated mobile station in that frequency block is applied.
On the other hand, adoption of the following method in a next-generation mobile communications system is under discussion, in which when a plurality of frequency blocks are allocated to each mobile station, a fixed MCS be applied per each of mobile stations over the frequency blocks.
In order to apply certain fixed MCS to a plurality of the frequency blocks while an MCS is changing, an MCS in a frequency block having a worst propagation characteristic cannot help being applied to all the frequency blocks so as to suppress the reception error rate within certain reference value or less. Accordingly, compared with a case where an individual MCS is applied to each frequency block per unit as in a conventional art, the transmission rate is likely to be more reduced. Consequently, frequency utilization efficiency may be reduced when the allocation method for the mobile station disclosed in Non-Patent Document 1 is applied to the next-generation mobile communications system without alteration.
In the example in FIG. 5, MCS6 can be applied to the frequency blocks #1, #2, and #12 to 14 among the frequency blocks allocated to the mobile station 1. However, MCS4, which is two levels lower than MCS6, needs to be applied to a frequency block #5. After all, it cannot be helped to apply the MCS4 in the frequency block #5 to the mobile station 1.
In order to determine a set (combination) of frequency blocks to be allocated to each mobile station that achieves the maximum system throughput while changing the combination of frequency blocks under a constraint condition that certain MCS is applied to a plurality of frequency blocks, a vast amount of computation is needed. The vast amount of computation is needed especially when the number of mobile stations and the number of frequency blocks are large.
When the frequency block #5 be allocated to the mobile station 2 instead of the mobile station 1 in the example in FIG. 5, MCS5 being one level higher than the MCS4 can be applied to the mobile station 1. Further, since the channel quality of the mobile station 2 in the frequency block #5 is the same as that of the mobile station 1, the number of bits transmitted through the frequency block #5 remains unchanged from that before the allocation is changed.
Accordingly, by changing the allocation, the total transmission rate achieved by all the mobile stations can be increased.
Further, with respect to the frequency blocks #3, #4, #10, and #11 as well, when the frequency blocks #3, #4, #10, and #11 be allocated to the mobile station 2 or 3 rather than the mobile station 1, the transmission rate obtained by summing up transmission rates of all the mobile stations may be more increased.
The number of alternatives for such allocation for each frequency block may be considered to be equal to the number of mobile stations. In other words, in the example in FIG. 5, it is necessary to consider 314 combinations. Thus, a vast amount of computation is necessary for achieving the optimum throughput of the system.
Accordingly, it is an object of the present invention to provide a radio resource allocation apparatus and a radio resource allocation method that can optimize system throughput with a small amount of computation, particularly, when one of a plurality of mobile stations is allocated to one or each of a plurality of frequency blocks, and when there is a constraint condition that certain constant MCS be applied per each mobile station.
According to a first aspect of the present invention there is provided a radio resource allocation method of allocating one of a plurality of mobile stations to one or each of a plurality of frequency blocks. The radio resource allocation method comprises:
(a) calculating allocation index representing transmission quality of each of the mobile stations in each of frequency blocks;
(b) extracting for each of frequency blocks, a mobile station having a largest allocation index as a candidate mobile station;
(c) extracting a mobile station having a largest allocation index in all of the frequency blocks, as an allocated mobile station for allocation; and
(d) extracting, among the frequency blocks, frequency block(s) for which the candidate mobile station matches the allocating mobile station, as candidate frequency block(s).
The method further comprises:
(e) sorting the candidate frequency blocks in descending order of magnitude of the allocation index;
(f) sequentially allocating the sorted candidate frequency blocks to the target mobile station by adding the sorted frequency block one by one, and also selecting a modulation and coding scheme applicable to the target mobile station; and
(g) calculating a transmission rate to be achieved by the target mobile station, based on the modulation and coding scheme and the number of the allocated candidate frequency blocks.
The method further comprises:
(h) allocating to the allocating mobile station a set of the candidate frequency blocks when the transmission rate has reached a transmission rate requested by the allocated mobile station or when a transmission rate has assumed a maximum value;
(i) excluding the allocated mobile station and the candidate frequency blocks allocated to the allocated mobile station, and (j) repeating the steps (b) through (i) until no allocation candidate mobile station or no allocation candidate frequency block is left.
According to a second aspect of the present invention there is provided a radio resource allocation apparatus of allocating one of a plurality of mobile stations to one or each of a plurality of frequency blocks. The radio resource allocation apparatus comprises:
an allocation index calculation unit that calculates an allocation index representing transmission quality of each of the mobile stations in the each of frequency blocks;
a candidate mobile station extraction unit that extracts, for the each of frequency blocks, a mobile station having a largest allocation index as a candidate mobile station;
a target mobile station extraction unit that extracts a mobile station having a largest allocation index in all of the frequency blocks, as a target mobile station; and
a candidate frequency block extraction unit that extracts, from among the frequency blocks, frequency block(s) for which the candidate mobile station matches the allocating mobile station, as candidate frequency block(s).
The apparatus further comprises:
a frequency block sorting unit that sorts the candidate frequency blocks in descending order of magnitude of the allocation index;
a modulation and coding scheme selection unit that sequentially allocates the sorted candidate frequency blocks to the target mobile station by adding the sorted frequency block(s) one by one, and also selects a modulation and coding scheme applicable to the target mobile station; and
a transmission rate calculation unit that calculates a transmission rate to be achieved by the target mobile station, based on the modulation and coding scheme and the number of the allocated candidate frequency blocks.
The apparatus further comprises:
a frequency block allocation unit that allocates to the target mobile station a set of the candidate frequency blocks when the transmission rate has reached a transmission rate requested by the target mobile station or when the transmission rate has assumed a maximum value; and
a control circuit that excludes the allocated mobile station and the candidate frequency blocks allocated to the allocated mobile station, and causes the candidate mobile station extraction unit, the candidate frequency block extraction unit, the frequency block sorting unit, the modulation and coding scheme selection unit, the transmission rate calculation unit, and the frequency block allocation unit to repeat operation until no allocation candidate mobile station or no allocation candidate frequency block is left.
The radio resource allocation method according to a first mode may comprise:
sorting in a higher order one of the candidate frequency blocks having an allocation index of a second largest magnitude which is smaller in the candidate frequency blocks, when candidate frequency blocks having the allocation index of a same magnitude are included in the sorting step (e).
In the radio resource allocation apparatus according to a second mode, when candidate frequency blocks having an allocation index of a same magnitude are included, the frequency block sorting unit is so configured that one of the candidate frequency blocks having an allocation index of a second largest magnitude which is smaller in the candidate frequency blocks is sorted in a higher order.
Meritorious effect of the present invention is mentioned below, however, without limitative active nature.
According to the radio resource allocation method or the radio resource allocation apparatus of the present invention, when one or each of the frequency blocks is allocated to one of the mobile stations and when a constraint condition that certain MCS is applied for each mobile station is present, system throughput can be optimized with a small amount of computation.